1. Field of the Invention
The present invention relates to a NOR-type nonvolatile semiconductor memory device and a manufacturing method thereof capable of decreasing a power supply voltage without causing write characteristics to deteriorate.
2. Description of the Related Art
Recently, demands for NOR-type flash memories have been rapidly increasing along with a rapid growth in markets of memory media including digital cameras and portable audio equipment including mobile telephones. At present, in order to achieve smaller sizes, lighter weights and enhanced functions of these devices, there are increasing requests for miniaturization, higher integration, lower power supply voltages and enhanced reliability in the NOR-type flash memories.
The NOR-type flash memory has a plurality of MOS are NOR-connected. As is well known, one element comprises a source diffusion layer and a drain diffusion layer oppositely formed on the surface of a semiconductor substrate, and a gate insulating film, a floating gate, an intergate insulating film and a control gate which are stacked on the semiconductor substrate between the source diffusion layer and the drain diffusion layer.
In contrast to a NAND-type flash memory which achieves writing by FN tunneling, the NOR-type flash memory implants hot electrons (HE) generated by use of electric field concentration at a drain edge into the floating gate to write information. At this point, because a voltage is applied to a drain electrode, there is caused a problem of deterioration of the gate insulating film located in the vicinity of the drain electrode. Thus, simple miniaturization of a gate length causes the deterioration of write characteristics.
While many reports have been made regarding problems and improvements in the size reduction of the NOR-type flash memory, a study has been conducted on the characteristics of writing achieved by secondary electrons induced in a channel, for example, in IEEE trans. Electron Devices, Vol. 50, No. 10, October 2003, pp. 2104-2111, wherein there are reported deterioration due to scaling (size reduction), the optimization of device parameters, trade-off for problems at the drain edge, etc.
However, it is known that write efficiency deteriorates as the gate length decreases, and simple miniaturization in conventional structures has already reached the limits. It has therefore been desired to achieve a NOR-type nonvolatile semiconductor memory device and a manufacturing method thereof capable of improving the write efficiency of the micro NOR-type flash memory.